AU2004211436A1

AU2004211436A1 - Method and device for pressure amplification in cylinders, in particular hydraulic rams

Info

Publication number: AU2004211436A1
Application number: AU2004211436A
Authority: AU
Inventors: Wolfgang Voss
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-02-14
Filing date: 2004-02-06
Publication date: 2004-08-26
Anticipated expiration: 2024-02-06
Also published as: DE502004004233D1; AU2004211436B2; US20060207251A1; EP1597484A1; ATE366370T1; EP1597484B1; DE10306128A1; WO2004072486A1; US7424803B2

Abstract

A method for increasing the working pressure in a working cylinder system has a concentric cylinder design and with the cylinder extended by applying pressure from a pump. Additional pressure is generated under the working piston (5) by a concentric smaller piston (16) which effectively reduces the volume under the working piston and thereby increases the effective pressure of the cylinder system. The smaller piston is supplied with fluid at the same pressure as the main piston. The design can be applied to cylinders operating with various fluids such as; hydraulic, hydraulic emulsion, pneumatic, plasma etc.

Description

Method and device for pressure amplification in cylinders, in particular hydraulic rams The invention concerns a method to increase the pressure in cylinders to be used 5 in oil hydraulics, water hydraulics, emulsion hydraulics as well as plasma and other fluid hydraulics and pneumatics, wherein two-part or multi-part cylinders are extended and retracted within one another in a controlled manner by means of valves and the pressure transmitter introduced, that has passed a high-pressure pump, while in the piston surface cavity below the respective piston a pressure, 10 specified also after the retraction, is produced and maintained. In addition, the invention concerns a cylinder to carry out the method according to claims 1 to 6, comprising a basic body with a cylinder housing that is connected to a high pressure pump via a supply line and a piston/piston rod displaceably provided in said cylinder housing and a piston surface cavity formed below the piston. 15 When actuating cylinders and similar apparatuses, the movement is affected by supplying a pressure transmitter, while one deals in this case with oil, water, water in oil emulsion, plasma or other liquids or also with air. Under hydraulics the teaching and the technical application of flows in compressible liquids is 20 understood. This means, that in hydraulics the liquid, above all oil or also water in oil emulsion is first correspondingly influenced, i.e. biased in a high-pressure pump so that to be conveyed to the cylinder via a hose or similar conduits. The extension and retraction of the cylinder is controlled by valves, while the work is carried out always with the one and same pressure, namely the one depending 25 on the output of the high-pressure pump. In particular in underground mining where, due to safety reasons, the work is carried out with water in oil emulsion, currently a maximum 400 bar is used, quite simply because currently there are no pumps capable to produce a higher pressure. Due to various reasons it is, however, not always assured, that the pressure mentioned would be available in 30 the piston surface cavity, for which reason especially in underground mining and tunnel construction so called post-charging circuits are known, with which the volume in the piston surface cavity is so adjusted by a repeated connection to the high-pressure pump, that approximately the aforementioned pressure will be available. However, it is not possible to generate a higher pressure level within the piston surface cavity and thus within the cylinder, because the aforementioned pumps are not capable to do this. This is, however, often desirable for the most varying reasons, while at the currently highest pressure level of 400 bar mentioned above the pumps represent the limit, at which lower 5 pressures would essentially negate the advantage of the expense for an additional high-pressure pump and corresponding hoses. Therefore the object of the invention is to produce a method and a device that enables the operation with cylinders with any internal pressure (working pressure) 10 that is arbitrarily increased relative to the pump pressure. According to the invention this objective is achieved by that a pressure transmitter (oil, water, emulsion, plasma, pressurised air) of a specified pressure is supplied first to the cylinder and the cylinder is extended with it and the pressure is then 15 arbitrarily increased in the piston surface cavity under the piston by using the pressure transmitter of the same pressure level for further compression of the pressure transmitter in the piston surface cavity. Thus this method makes it possible to increase this independently from the initial 20 pressure level, to achieve advantages in this manner. For example, starting with a pressure clearly below 400 bar, with the aid of the method according to the invention it is possible to deliberately increase the pressure level in the cylinder, without the necessity of other conduits, a cylinder with different dimensions or the like. Rather will the pressure be so increased in the cylinder, that the desired 25 pressure will be achieved. By virtue of this in underground mining and tunnel construction, for example, the required setting pressure can be achieved in the ram, i.e. in the correspondingly constructed cylinder, without the necessity to use the expensive high-pressure pump that also requires special conduits. Instead of this a cost-effective and simpler high-pressure pump, achieving correspondingly 30 lower pressures, can be used yet reach a high pressure level within the cylinder or ram by using the method. It is, of course, also possible for the first time pass the pressure of 400 bar, because in the case of a corresponding charging of the cylinder with this pressure transmitter a markedly higher pressure can be generated within the cylinder to achieve the desired task. Thus it is possible, for 3 example, to achieve with the invention a targeted high pressure or to cover a greater path. The cylinders do not need to be modified. However, if in contrast, as it is explained in the following, one commences with a lower pressure level in the region of the ram, but above all in the hoses simpler wall thicknesses and 5 executions can be used to reduce the investments. It is now also possible to use, especially in underground mining and tunnel construction, any type of plug-in connector even when operating with higher pressures. On the other hand one can operate with a lower pressure in hoses with a larger diameter so that to achieve in this manner the quantity of the pressure transmitter to be conveyed or 10 set it high. A useful development of the invention provides, that the pressure transmitter is supplied to the cylinder from a pump operating at 200 bar and the pressure is increased in the piston surface cavity to 300-400 bar. This will be achieved by a 15 previously mentioned transmission system that will be explained in the following, with the aid of which the pressure level within the cylinder can be increased correspondingly simply and reliably and depending on the dimensions. A development provides, that the volume of the piston surface cavity is reduced 20 by supplying the pressure transmitter and consequently a higher compression of the pressure transmitter is produced. Therefore the pressure transmitter will be further compressed without the necessity to supply the pressure transmitter, therefore an oil or pressurised air at a higher level, i.e. no special or other pump and a correspondingly other system pressure is required. This has the already 25 mentioned advantages, but has also the additional advantage that it can be realised simply and without any problem. The invention wants to achieve the increase of the pressure level within the cylinder, what according to the invention is achieved by that a piston with smaller 30 dimensions is pushed into the piston surface cavity over the pressure transmitter of the same pressure level, thus compressing the pressure transmitter therein. Due to this the pressure level is greatly increased and to the extent that is required for the relevant application.

4 With regard to the method, the invention provides, depending on the field of application, that the pressure transmitter of the same pressure level is switched on depending on other switching operations or separately, so that to hold the possibly required switching effort as low as possible. 5 In turn, depending on the field of application and the area of application, the invention makes it possible, that the switching on of the pressure transmitter of the same pressure level is carried out in series or parallel, what is possible, inter alia, by that a second system pressure is not available and is not required either 10 for the purpose of increasing the pressure or the pressure level under the piston. For the carrying out of the method a cylinder is provided, that comprises a cylinder housing and a piston with a piston rod that can be displaced therein, while the piston surface cavity is formed below the piston, the pressurised fluid or 15 the pressure transmitter is introduced into this cavity to achieve the extension of the cylinder. In this conjunction a modifiable pressure level is achieved within the piston surface cavity by that the piston surface cavity, in fact the pressure transmitter situated therein, can be influenced with the same high-pressure pump directly via an inlet valve and additionally indirectly via a compressor. It has 20 already been explained, that in the case of the compressor one deals with a transmission system that assures in this manner that the greater pressure in the piston surface cavity can be actually achieved even when the pressure transmitter used and required for this purpose has a correspondingly lower pressure level. Thus by this present invention it is possible for the first time to 25 modify the pressure level within the piston surface cavity, first of all, of course, to increase it, in fact above the values achievable with currently used high-pressure pumps. A useful embodiment of the invention is that, wherein the piston surface cavity is 30 constructed as a bore executed in the cylinder cover and the piston as well as in the piston rod, whereby the bore in the piston rod is so expanded that it accommodates a bush, in which bush a collet piston, acting as a compressor and having a small piston rod, the diameter of which corresponds to the bore, is displaceably provided and at its head can be connected via a connecting valve 5 with the same high-pressure pump. The number of the single parts of the transmission system already makes it clear, that one faces here a solution that leads to an aim in a surprisingly simple manner, that so far had not even been considered. It is possible, without changing the pressure level of the high 5 pressure pump to be used, so to increase the pressure level within the piston surface cavity, i.e. within the cylinder, that with it additional tasks can be carried out, in fact without the necessity of modifying the device. The piston surface cavity is connected with the high-pressure pump, once directly 10 and once indirectly, whereby it is provided that the piston surface cavity can be directly connected with the high-pressure pump via the inlet valve provided at the head of the piston. Figuratively speaking, with regard to the underground application the inlet valve is therefore situated in the foot region of the ram or of the cylinder, so that the piston with the piston rod will be reliably extended when 15 the connection with the high-pressure pump is established, because the load will act on a large area of the piston. The collet piston, displaceably provided within the cylinder, is in turn visually , and using the example of an underground ram, will be charged from above with 20 pressurised fluid, i.e. with the pressure transmitter. The invention provides, that the bush, accommodating the collet piston, is closed by a rod head that also closes the large piston rod, to which connecting bores, connecting the connecting valve and the collet piston on the side of the head, are allocated. This rod head is partly pushed into the bush and the corresponding large piston rod and fixed 25 therein, while it will be held in this position merely by that it is clamped in or loaded from above. The connecting valve rises, therefore, with the rod head, but it will be always easily accessible, if only because it laterally protrudes past the rod head and the piston head. Thus it will be ensured, that as far as the pressure level is concerned, the additional pressure can be produced in the cylinder with 30 the one and same pressure transmitter. The pressure transmitter is conveyed to the side of the head via the connecting valve and the connecting bores to the collet piston, so that it will be correspondingly displaced forced by the specified areas. The collet piston has corresponding dimensions, it is considerably larger than the piston rod.

b The rod head, that closes the bush at the top and consequently also the hollow large piston rod, ensures that the pressure conditions within the cylinder can be correspondingly built up. 5 To ensure a corresponding compression in the piston surface cavity, it is provided that on the side of the head the collet piston has a plate-shaped recess. This plate-shaped recess ensures, that the pressurised fluid or the pressure transmitter can act immediately on an as large as possible area of the piston. It also prevents that the collet piston would remain hanging on the rod head. Thus 10 altogether a fast, reliable response of the corresponding transmission system is assured. Reference has been made earlier to the fact, that the collet piston has a diameter that is considerably greater that that of the associated small piston rod. In 15 particular the piston rod has a diameter of 68 mm and the collet piston a diameter of 110 mm. In contrast the piston surface cavity has a diameter of 70 mm, what will be explained later. According to the above mentioned dimensions an annular gap remains 20 deliberately, while the invention provides that the small piston rod is so executed that an annular gap remains between it and the wall of the bore of the piston, said annular gap being connected with the underside of bottom of the bush that can be passed through by the small piston rod. By virtue of this, when the small piston rod is pushed into the bore, pressurised fluid is displaced through the annular 25 gap, taking care of that the bottom of the bush is correspondingly loaded, so that the bush will be displaced in the longitudinal direction of the cylinder, carrying the rod head with it. A useful development provides, that the bottom of the bush has a stepped 30 construction, whereby the lower small step has sealing rings effective against the outside wall of the small piston rod and the inside wall of the large piston rod. Whereas in the first instance only a small surface is available for the further compressed pressure transmitter, this surface will become greater when the bush is lifted out from the second seat, so that the second step will become effective.

t It becomes obvious from the above mentioned measures, but also from the basic effect, that the collet piston has a greater diameter than the small piston rod, thus having an annular air space in the bush, in which air is present. When the collet piston and the small piston rod is pushed in, the air existing therein has to be able 5 to be displaced, what is achieved by that in the region of the bottom of the bush the bush has a transverse bore connecting the annular air gap between the inside wall of the bush and the outside wall of the small piston rod with a longitudinal bore leading to the atmosphere. Thus when the collet piston and the small piston rod moves in, the air will be pressed without any problem through the transverse 10 bore into the longitudinal bore and from there it can escape into the atmosphere. In a reverse process air will penetrate from the atmosphere and the longitudinal bore as well as the transverse bore into the annular air space, so that the collet piston and the small piston rod can be reliably moved back to the initial position. 15 To retract the cylinder again, the large piston is charged from below by the pressure transmitter or the pressurised fluid. This is achieved in particular usefully by that between the inside wall of the cylinder housing and the outside wall of the large piston rod an annular space is formed, that is enclosed downward by the piston with the sealing rings and upward by a locking ring with integrated sealing 20 rings, said annular space being connectable via a retraction valve with the same high-pressure pump. Therefore, when the connection with the high-pressure pump is established via the retraction valve, the pressure transmitter penetrates directly into the annular space, while the sealing rings ensure that the annular space is sealed to the outside. Because of this the pressure transmitter acts 25 exclusively on the underside of the large piston, so that it will be pushed back to its initial position. To reliably position the locking ring, it is provided that the locking ring has an outside thread that corresponds with the inside thread associated with the end of 30 the cylinder housing. Thus the locking ring can be correspondingly screwed in and will be in a secure position even when the cylinder is extended or retracted. Despite the quite high pressures built up in the interior of the cylinder, it is sufficient when the sealing rings, allocated to the locking ring, are arranged in a b sealing manner against the large piston rod and against the annular space. In an appropriate manner the locking ring has on the side of the piston rod two sealing rings, and a third sealing ring facing downwards, i.e. in the direction of the annular space. 5 The invention is particularly characterised in that a method and a device are produced, with which the operation of cylinders is considerably simplified. One can work either with pumps of lower outputs, in which case the required pressure increase is carried out in the cylinder or with the same units, in particular with 10 high-pressure pumps producing the highest pressures and then produce inside the cylinder a considerably higher pressure, that results either in a longer path or higher pressure within the cylinder. In an overall view, with the aid of the method and the device according to the invention the technical world will be able to simplify existing hydraulic systems or compressed air systems, make do with 15 cost-effective and thin-walled hoses, and possibly also with corresponding cylinders or operate at such a high pressure level, that the previously mentioned advantages will be achieved with it, while current hoses can be utilised. Further details and advantages of the subject matter of the invention become 20 apparent from the following description of the associated drawing, in which a preferred embodiment is illustrated with the details and individual parts necessary for it. They show in: Fig.1 - a longitudinal section through a cylinder with the transmission system, 25 Fig.2 - a shield construction with a plurality of cylinders to be used in underground mining, and Fig.3 - an excavator, that is also equipped with a plurality of cylinders. 30 Fig. 1 shows a longitudinally sectioned cylinder 1, from which it will be clear that the base body 2 of this cylinder 1 comprises the cylinder housing 3 with the cylinder cover 4 and the piston 5 with the piston rod 6. The piston 5 with the piston rod 6 is displaceably arranged within the cylinder housing 3, while the supply line 7 introduces pressurised fluid or the pressure transmitter into the piston surface cavity 8 via the inlet valve 9. This pressure transmitter, that penetrates into the piston surface cavity 8, ensures that the piston 5 with the piston rod 6 and with the rod head 19 at the end move out of the cylinder housing 5 3. The interior, designated here as compressor 10, does not prevent this moving out and supports it. In the example illustrated the piston surface cavity 8 is formed by the bore 11 in the cylinder cover 4, the bore 12 in the piston and the bore 13 in the piston rod 6. 10 Thus the piston surface cavity 8 has the shape of a cylinder. As already mentioned, the large piston rod 6 has a hollow construction, while the bore 13 extends up to the upper end. In this bore 13 a bush 15 is provided, that accommodates a collet piston 16 with the small piston rod 17. The collet piston 15 16 can be displaced in the longitudinal direction, while it can be connected to the supply line 7' via a connecting valve 18 and the connecting bores 20, 21 and with the high-pressure pump not illustrated here. Thus the pressure transmitter is conveyed from the supply line 7 to the connecting valve 18 and from there via the two connecting bores 20, 21 to the collet piston 16, that has a plate-shaped 20 recess 22 in this region. This will ensure, that the inflowing pressure medium, i.e. the pressure transmitter will act on the full area of the collet piston 16. On the edge of the collet piston 16 seals 23, 24 are provided, taking care of the required sealing and ensuring that the pressure transmitter acts on the collet piston 16 right up to the bush 15. The correspondingly large diameter of the collet piston 16 25 then ensures, that in the case of a corresponding load by the usual pressure transmitter, i.e. at pressure levels usually achieved by the pressure transmitter, the collet piston 16 and the small piston rod 17 are displaced in the direction of the piston surface cavity 8. By doing so, the small piston rod 17 displaces the pressure transmitter situated in the piston surface cavity 8 and initially 30 compresses it. Only when an additionally increased pressure occurs in the piston surface cavity 8, will the pressurised liquid or pressure transmitter act on the underside 29 of the bottom 30 of the bush via the annular gap 26 between the wall 27 of the bore of the piston 5 and the outside wall 28 of the small piston rod 17. This effect or influence will be increased when due to the pressure conditions 1u the bush 15 with the rod head 19 has moved, because the bottom 30 of the bush has two steps 31, 32. In the bottom 30 of the bush sealing rings 34, 35 are so arranged, that they seal against the inside wall 33 of the large piston rod 6 on the one hand and against the outside wall 28 of the small piston rod 17 on the other. 5 It can be seen from Fig.1, that due to the different dimensions of the collet piston 16 and of the small piston rod 17, an annular air space 37 remains between the latter and the inside wall 38 of the bush, said annular space is connected via a transverse bore 40 with a longitudinal bore 39, that leads to the atmosphere since 10 it exits at the end of the bush 15. This provides the possibility for the air to escape from the annular air space 37 through the transverse bore 40 and the longitudinal bore 39 during the inward movement of the collet piston 16. The other way around, during the return movement of the collet piston 16 with the small piston rod 17, air can return in the reversed direction, i.e. through the longitudinal bore 15 39 and the transverse bore 40 into the annular air space 37. For the inward movement of the piston 5 with the large piston rod 6 an annular space 45 is provided between the inside wall 43 of the cylinder housing 3 and the outside wall 44 of the large piston rod 6. This annular space 45 extends down up 20 to the piston 5. The annular space 45 is connected with the retraction valve 52 via a transverse bore 42, so that when required pressure transmitter can be brought into this annular space 45 via the supply line 7". This pressure transmitter ensures that due to the relief in the region of the inlet valve 9 the piston 5 with the large piston rod 6 is pushed back into the initial position shown in Fig.1. To 25 ensure in this conjunction the effect of the pressure transmitter, sealing rings 46, 47 are provided in the piston 5, that seal against the annular space 45 on the one hand and against the inlet valve 9 on the other. At the end 55 of the cylinder housing the annular space 45 is joined via a locking 30 ring 48 that has several sealing rings 49, 50, 51 for the purpose of sealing against all sides. It has an outside thread 54, that has a construction corresponding to the inside thread 56 of the end 55 of the cylinder housing, so that it can be screwed in.

11 Fig.2 shows a longwall construction 60, that is fitted with a plurality of cylinders, to be explained in the following. It is especially in the underground mining and tunnel construction where water in oil hydraulics are used as a safety standard, that is excellent with respect to the fluid used and the setting forces required. The 5 conveyed material extracted is conveyed via the conveying means 61, while this conveying means 61 extends in the direction of the background and in reverse and is connected with the base plate 63 of the longwall construction 60 via a thrust cylinder 62. The conveying means 61 is always influenced via the thrust cylinder 62 in such a manner, that it will be placed tightly against the coal face, 10 not illustrated here. The face shield 64 is joined to the base plate 63 by means of a plurality of lemniscate rods 65, so that to always occupy an optimum position relative to the floor. The ram(s) 66, a multi-part ram in this case, are supplied with a hydraulic fluid at an approx. 360-400 bar pressure via a supply line extending along the longwall. These rams 66 are connected, as this will be explained later, 15 via the connecting valve 67 directly and indirectly with the high-pressure pump. For this purpose only one supply line is required, the construction of which can be particularly simplified when the cylinder, i.e. the ram 66 as well as the other cylinders are constructed as shown in Fig.1. The roof cap 68, supported by the ram 66, is joined to the face shield 64. The front portion of this roof cap 68 forms 20 the sliding part 69, that can be also extended and retracted by a cylinder (not illustrated here) to enable to move the tip 72 of the roof cap 68 as far as possible up to the coal face. Such a longwall construction extends between the roof 70 and the floor 71 and ensures that the hollow space would remain open as long as this is necessary for the extraction. 25 Fig.3 shows simplified a hydraulic excavator 73 with its travelling undercarriage 74 and the swivelling arm 75. At the same time cylinders are allocated to the swivelling arm 75 as well as to the shovel 77 and other components, by means of which the individual functions can be carried out or made easier. The lower part 30 of the swivelling arm 75 is raised by the setting cylinder 76 or brought to another position, whereas the shovel 77 is controlled in its movement by the shovel cylinder 78. The application of this present invention is of particular interest in such hydraulic excavators 73 as well as in the longwall construction 60, because a plurality of cylinders are used in these cases, which have various tasks. For this reason it is not absolutely necessary to equip all cylinders with an internal compressor. All features mentioned, also those which become apparent only from the 5 drawings, are considered alone and in combination as essential for the invention.

Claims

1. A method to increase the pressure in cylinders to be used for oil hydraulics, water hydraulics, emulsion hydraulics as well as plasma and other fluid 5 hydraulics and pneumatics, wherein two-part or multi-part cylinders are extended and retracted within one another in a controlled manner by means of valves and the pressure transmitter introduced, that has passed a high pressure pump, while in the piston surface cavity below the respective piston a pressure, specified also after the retraction, is produced and maintained, 10 characterised in that a pressure transmitter (oil, water, emulsion, plasma, pressurised air) of a specified pressure is supplied first to the cylinder and the cylinder is extended with it and then the pressure is arbitrarily increased in the piston surface cavity under the piston by using the pressure transmitter of the same pressure level for further compression of the pressure transmitter in the 15 piston surface cavity.

2. A method according to claim 1, characterised in that the pressure transmitter is supplied to the cylinder from a pump operating at 200 bar and the pressure is increased in the piston surface cavity to 300-400 bar. 20

3. A method according to any one of the preceding claims, characterised in that the volume of the piston surface cavity is reduced by supplying the pressure transmitter and consequently a higher compression of the pressure transmitter is produced. 25

4. A method according to any one of the preceding claims, characterised in that a piston with smaller dimensions is pushed into the piston surface cavity over the pressure transmitter of the same pressure level, thus compressing the pressure transmitter therein. 30

5. A method according to any one of the preceding claims, characterised in that the pressure transmitter of the same pressure level is switched on depending on other switching operations or separately. 14

6. A method according to claim 5, characterised in that the switching on of the pressure transmitter of the same pressure level is carried out in series or parallel. 5

7. A cylinder to carry out the method according to claim 1 and/or one of the subsequent method claims, comprising a basic body (2) with a cylinder housing (3) that is connected to a high-pressure pump via a supply line (7) and a piston/piston rod (5, 6) displaceably provided in said cylinder housing and a piston surface cavity (8) formed below the piston (5), characterised in 10 that the piston surface cavity (8), in fact the pressure transmitter situated therein, can be influenced with the same high-pressure pump directly via an inlet valve (9) and additionally indirectly via a compressor (10).

8. A cylinder according to claim 7, characterised in that the piston surface cavity 15 (8) is constructed as a bore (11, 12, 13) executed in the cylinder cover (4) and the piston (5) as well as in the piston rod (6), whereby the bore (13) in the piston rod (6) is so expanded that it accommodates a bush (15), in which bush a collet piston (16), acting as a compressor (10) and having a small piston rod (17), the diameter of which corresponds to the bore (11, 12, 13), is 20 displaceably provided and at its head can be connected via a connecting valve (18) with the same high-pressure pump.

9. A cylinder according to claim 7, characterised in that the piston surface cavity (8) can be directly connected with the high-pressure pump via the inlet valve 25 (9) provided at the head of the piston (5).

10. A cylinder according to claim 8, characterised in that the bush (15), accommodating the collet piston (16), is closed by a rod head (19) that also closes the large piston rod (6), to which connecting bores (20, 21), 30 connecting the connecting valve (18) and the collet piston (16) on the side of the head, are allocated.

11. A cylinder according to claim 10, characterised in that on the side of the head the collet piston (16) has a plate-shaped recess (22).

12. A cylinder according to claim 10, characterised in that the collet piston (16) has a diameter that is considerably greater that that of the associated small piston rod (17). 5

13. A cylinder according to claim 8, characterised in that the small piston rod (17) is so executed that an annular gap (26) remains between it and the wall (27) of the bore of the piston (5), said annular gap being connected with the underside (29) of bottom (30) of the bush that can be passed through by the small piston rod (17). 10

14. A cylinder according to claim 13, characterised in that the bottom (30) of the bush has a stepped construction, whereby the lower small step (31) has sealing rings (34, 35) effective against the outside wall (28) of the small piston rod (17) and the inside wall (33) of the large piston rod (6). 15

15. A cylinder according to claim 12, characterised in that in the region of the bottom (30) of the bush the bush (15) has a transverse bore (40) connecting the annular air gap (37) between the inside wall (38) of the bush and the outside wall (28) of the small piston rod (17) with a longitudinal bore (39) 20 leading to the atmosphere.

16. A cylinder according to any one of the preceding claims, characterised in that between the inside wall (43) of the cylinder housing (3) and the outside wall (44) of the large piston rod (6) an annular space (45) is formed, that is 25 enclosed downward by the piston (5) with the sealing rings (46, 47) and upward by a locking ring (48) with integrated sealing rings (49, 50, 51), said annular space being connectable via a retraction valve (52) with the same high-pressure pump. 30

17. A cylinder according to claim 16, characterised in that the locking ring (48) has an outside thread (54) that corresponds with the inside thread (58) associated with the end (55) of the cylinder housing. Ilb

18. A cylinder according to claim 16, characterised in that the sealing rings (49, 50, 51), allocated to the locking ring (48), are arranged in a sealing manner against the large piston rod (6) and against the annular space (45). 5